1. Field of the Invention
The present invention relates to an automatic equalizer for use in a digital radio communication system, and more particularly to an automatic equalizer composed of an adaptive matched filter and a decision feedback equalizer, for eliminating intersymbol interference due to multipath fading in radio communications.
2. Description of the Related Art
One problem with conventional digital radio communication systems has been the degradation of radio circuit quality due to frequency selective fading that occurs in the transmission path. To solve this problem, there has been proposed a digital radio communication system including a receiver which is composed of an adaptive matched filter and a decision feedback equalizer (see K. Okanoue et al.: "New Equalizing System for Super Multilevel QAM", IEICE 1989 Spring National Convention Record, B-929, the Institute of Electronics, Information and Communication Engineers).
If the ratio of the amplitude of a delayed wave, which is a reflected wave, to the amplitude of a preceding wave, which is a direct wave, is represented by .rho., then the decision feedback equalizer has fading equalizing characteristics for two-wave interference such that when .rho.&lt;1, it can sufficiently equalize intersymbol interference, but, when .rho.&gt;1, its fading equalizing characteristics are poorer than when .rho.&lt;1. It is known that the equalizing capability of the decision feedback equalizer is low in a case in which a preceding wave arrives earlier than a direct wave (see M. Muroya and H. Yamamoto: Chapter 6 of "Digital Radio Communications"0 published by Sangyo-Tosho Co., Ltd. 1985).
To improve the equalizing characteristics at the time .rho.&gt;1, it has been proposed to place the adaptive matched filter so as to precede the decision feedback equalizer.
FIG. 1 of the accompanying drawings shows a fundamental circuit illustrative of the operating principles of the adaptive matched filter.
For transmitting pulses over a band-limited transmission path without causing intersymbol interference in digital radio communications, the impulse response of the entire transmission path must usually be zero, except for the central peak, at each interval T, as shown in FIG. 2(A) of the accompanying drawings. However, when the transmission path includes two paths, one for a direct wave and one for a reflected wave, these waves interfere with each other, causing multipath fading. If the amplitude of the reflected wave is greater at this time than the amplitude of the direct wave, i.e., .rho.&gt;1, the reflected wave becomes a principal wave, and the direct wave that arrives earlier than the reflected wave becomes an interfering wave, with the result that a large intersymbol interference is produced when t=-T as indicated by the impulse response of the transmission path as shown in FIG. 2(B) of the accompanying drawings. In FIG. 3(A) of the accompanying drawings, the principal wave r.sub.0 (=a (m)) and the intersymbol interference r.sub.-1 (=a(m+1)) at the time t=-T are indicated by the corresponding arrows. When the principal wave r.sub.0 and the intersymbol interference r.sub.-1 are delayed by T, they are indicated by the corresponding arrows in FIG. 3(B) of the accompanying drawings.
For the sake of brevity, a 2-tap transversal filter 102 comprising a delay circuit 6, multipliers 7, 8, and an adder 9 as shown in FIG. 1 will be considered. An input signal S.sub.0 is represented as shown in FIG. 3(A), and a signal S.sub.1 is represented as shown in FIG. 3(B). At this time, an output signal S.sub.2 from the adder 9 can be given as follows: EQU S.sub.2 =.alpha..times.S.sub.0 +.beta..times.S.sub.1.
If .alpha.=0.9/1.9 and .beta.=-1/1.9, the output signal S.sub.2 can be expressed by: EQU S.sub.2 =(0.9/1.9).times.S.sub.0 +(-1/1.9).times.S.sub.1.
Therefore, the output signal S.sub.2 is represented as shown in FIG, 3(C). Consequently, after the input signal S.sub.0, which has been subjected to the large intersymbol interference r.sub.-1 due to the leading wave as shown in FIG. 3(A), has passed through the transversal filter 102 with the tap coefficients .alpha., .beta., the intersymbol interference is dispersed into intersymbol interferences r.sub.-1, r.sub.1 that are symmetric with respect to the principal signal r.sub.0 (t=0). It should be noted that the magnitude of the dispersed intersymbol interferences r.sub.-1, r.sub.1 is about half of the magnitude of the intersymbol interference r.sub.-1 before it is dispersed.
Consequently, while the decision feedback equalizer alone, denoted at 201 in FIG. 1, is unable to equalize the intersymbol interference r.sub.-1 =0.9 of the signal S.sub.0, the combination of the decision feedback equalizer 201 and the transversal filter 102 that precedes the decision feedback equalizer 201 can sufficiently equalize the intersymbol interference of the above magnitude because the intersymbol interference r.sub.-1 is of a magnitude r.sub.-1 =0.9/1.9=0.47 and the intersymbol interference r.sub.1 is of a magnitude r.sub.1 =1/1.9=0.53. The transversal filter which functions in the manner described above is also referred to as a matched filter. Thus, the adaptive matched filter and the decision feedback equalizer in combination are capable of improving equalizing characteristics when .rho.&gt;1.
The automatic equalizer is reset intermittently so that the control loop in the automatic equalizer of the above arrangement converges when the control loop diverges.
FIG. 4 of the accompanying drawings illustrates a conventional automatic equalizer. An analog baseband signal supplied from a demodulator (not shown) is ,inputted from an input terminal 1, and sampled and quantized into a first digital signal series by an analog-to-digital (A/D) converter 4. The first digital signal series is supplied to an adaptive matched filter 101, which supplies a second digital signal series with symmetrized impulse responses to a decision feedback equalizer 201. The decision feedback equalizer 201 outputs a third digital signal series free of intersymbol interference from an output terminal 3.
An asynchronism-detected signal supplied from the demodulator is inputted from the input terminal 2 to a reset control circuit 302, which outputs a reset signal C. During a synchronous period in which the control loop converges, the reset signal C controls the adaptive matched filter 101 and the decision feedback equalizer 201 to operate in an automatic equalizing mode. During an asynchronous period in which the control loop diverges, the reset signal C intermittently resets the adaptive matched filter 101 and the decision feedback equalizer 201, as shown in FIG. 5 of the accompanying drawings. Therefore, if the control loop cannot converge when the adaptive matched filter 101 and the decision feedback equalizer 201 are brought into the automatic equalizing mode for a certain time in the asynchronous period, the adaptive matched filter 101 and the decision feedback equalizer 201 are reset and then returned to the automatic equalizing mode again. Such a process is repeated until the synchronous condition is reached. See Japanese patent laid-open No. 196713/1983.
In the conventional automatic equalizer, as described above, when the control loop diverges, the intermittent reset signal is sent to the adaptive matched filter 101 and the decision feedback equalizer 201 at the same time. Accordingly, when the control loop diverges under the condition .rho.&gt;1, the distortion is so large for the decision feedback equalizer 201 that it cannot effect proper control.
If the control speed of the decision feedback equalizer 201 is first lowered in order to converge the adaptive matched filter 101, the converging characteristic of the decision feedback equalizer 201 is degraded. In any case, it is difficult to converge the control loop upon fading.